Continuous research and development in the IC (integrated circuits) industry has led the fabrication technology of semiconductor chips into the ULSI (ultra large scale integration) stage. The integrity or packing density of the semiconductor devices increases at an exponential rate. A single chip may contain millions or even hundreds of millions of devices. The size of the devices on the semiconductor substrate has become smaller and smaller for packing into a chip with a high density.
For example, the capacity of a single DRAM (dynamic random access memory) chip has increased from 16 megabit and 64 megabit to 256 megabit or even larger capacity. The size of devices such as transistors, connections, and capacitors have to be narrowed for several times to realize the increased capacity within almost the same chip size. The major challenge is to manufacture every element in a much smaller size without degradation in functionality.
Capacitors are vital elements in lots of applications like DRAM, memory arrays, logic circuits, and analog circuits. The structure of a capacitor is formed by sandwiching two conductive layers with a dielectric layer in-between. The ability of a capacitor to hold electric charge, namely the capacitance, is proportional to the surface area of the conductive layers. In a densely packed chip, the capacitor must maintain the capacitance with reduced size, or the operational characteristics of the memory cell or the circuits can be damaged. Under the challenge, many methods and technologies were proposed to fabricate capacitors in smaller size without reduced capacitance. The technology of a rugged silicon film or hemi-spherical grain (HSG) polysilicon film is widely employed for its large effective surface area within limited region.
A polysilicon layer is formed to have a rugged surface or a surface fill of hemi-spherical grain to increase the surface area significantly. In general, the HSG silicon film is formed on a conductive layer like doped polysilicon to serve as first electrodes of capacitors. An etching-back step must be performed to define the individual electrodes on the substrate. However, the HSG silicon film can be easily damaged by the etching-back step to the widely distributed small grains. The electrical characteristics, the shape, and the surface area of the film can be greatly influenced by the etching-back induced damage. Thus a capacitor with a damaged electrode surface can suffer from problems of decreased capacitance and dielectric layer leakage. The reliability and functional characteristics of the device and the circuit can be spoiled.
What is needed is a method for forming HSG films which are free of etching-back induced damage, in order to increase the capacitance of the capacitors and reliability of the devices.